Along with the increased speeds and frequencies of computers, communications equipment, and other similar devices in recent years, the wiring boards used in the installation of electronic parts have undergone a lowering of their dielectric constants and an increase in density. As to the former, substrates made of fluorine resins have been used because of their low dielectric constants. As for the latter, ceramic substrates have been used because of the advantages in their manufacturing methods and material characteristic.
Low dielectric constant substrates in which the above-mentioned fluorine resins are utilized are reinforced with glass cloth, polyimides, or other materials with high dimensional stability, and therefore have quite good dimensional stability themselves, but the wiring pattern can only be made so dense. They do not, therefore, allow the wiring density to be increased very much, and as a consequence, there is a limit to how much the wiring spacing can be reduced. On the other hand, materials in circuit boards made from ceramic have a high level of dimensional stability, and the wiring pattern can be made fine, i.e., more dense, so that a higher density can be realized. However, there is a drawback in that because the material itself has high dielectric constant, the transmission speed of the signal is slower. In order to compensate for this disadvantageously high dielectric constant, a porous ceramic is desirable so as to cause the material to contain air, thereby lowering the dielectric constant; but the mechanical strength of porous ceramic is weak, and the substrate cannot be used as it is.
Thereupon, the objective of the present invention is to offer a ceramic substrate which permits high density wiring at a low dielectric constant.